Published on Dec 29, 2019

Space Star Background Images

License Info: Creative Commons 4.0 BY-NC

Stars are familiar objects. We see them every day and every night. A single star, our Sun, makes it possible for life on earth to thrive. Stars live so long that they seem eternal to us. But they all had a beginning, and they’ll all have an end, too.

Stars begin their lives in immense clouds of gas and dust called nebulae. Nebulae contain mostly hydrogen gas, with a little helium. In most places, a nebula is often so gossamer-thin that if you walked through one, you’d never notice. However, there are some areas in a nebula where gravity causes the gas and dust to coalesce, getting more and more dense and hot.

As more and more material clumps together in these areas, the temperature inside can get extremely hot–around fifteen million degrees. The gravitational pressure is also so immense that the electrons within the atoms of hydrogen and helium are stripped, leaving only the nuclei. As the gravitational force increases, the atomic nuclei fuse. This event is called nuclear fusion. It creates an enormous outpouring of energy strong enough to counteract the crushing gravitational force. The energy pours out into space as light, heat, and electromagnetic radiation. A new star is born, and it begins to shine.


Throughout its life, a star is caught in a balancing act between the crushing forces of gravity and the explosive forces of nuclear fusion within its core. As long as there is material within the star’s core that can undergo the process of nuclear fusion, the star will fight gravity and continue to shine. But when the star runs out of fuel, it dies.

Most stars are the size of our sun or smaller. Our sun is a yellow dwarf star, and it’s fairly average within the universe. Stars the size of our sun usually emit light in the yellow or orange spectrum, and they live for about ten billion years.

The process of nuclear fusion converts hydrogen into helium. A star the size of our sun turns all its hydrogen into helium, and then starts converting helium into oxygen and carbon as it ages. The oxygen we breathe, and the carbon that provides the most crucial material for life on earth, was created inside a star like our sun.

As a medium-sized star ages, the central core contracts and the outer envelope expands to many times the star’s original size. This changes the star from a yellow dwarf to a red giant. This will happen to our sun someday; it will swell until it encompasses even Jupiter’s orbit. Red giants are many times larger than our sun, but they are also cooler.

Eventually, the core contracts as much as possible. The atoms within are packed so tightly that they cannot be further contracted. When that happens, a large amount of energy is released, ripping away the outer envelope of gas and forming a nebula of gas and dust.

The only thing left of the star is the tightly-packed core, called a white dwarf star. White dwarfs still emit heat and light, but they no longer undergo nuclear fusion. Eventually, when they radiate all their heat, they will become cold, lifeless black dwarf stars. There are no black dwarf stars today; the universe has not existed long enough for any white dwarf star to lose all of its heat.

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